Some of the most familiar galls in eastern North America are the spherical ones on goldenrod, which are caused by a fruit fly, Eurosta solidaginis (Tephritidae). (Tephritid fruit flies have nothing to do with the tiny “fruit flies” common in kitchens, which are in the family Drosophilidae.)
Each gall contains a single plump, juicy larva, which overwinters inside the gall. Downy woodpeckers and chickadees often peck holes in these galls in the winter to go after this tasty morsel. I collected five goldenrod ball galls on March 20, and on April 25 flies emerged from two of them–one male and one female.
When I had reasonable indoor shots of the flies, I took them outside for some more natural-looking shots:
There is a whole website devoted to these galls and their ecology here, and this page on BugGuide.Net summarizes the gall makers, parasitoids, and other insect associates of these and other goldenrod galls.
(Added May 3) Here is a third fly, with partially deflated ptilinum, which didn’t make it all the way out of its gall: